Laser Diodes for Holography
A diode laser is an interesting beast for holography. It has long coherence length but the frequency dependence on temperature is extremely critical.
From Tom B.
As a first approximation, coherence length = (wavelength ^ 2) / (2 * linewidth)
e.g. for 670 nm center wavelength and line width (wavelength range) of 0.2 nm,
coherence length = (670 * 10E-9)^2 / 2*(0.2E-9) = 0.0011 meter
Exact value would depend on the shape of the line.
The frequency equivalent:coherence length = speed of light / bandwidth
e.g. for 1500 MHz, this is 3E8 / 1500E6 = 0.2 meter.
(Iovine's book had a typo in this equation, but his example was correct)
Math and example from Iovine, "Homemade Holograms", 1990.
The rule of thumb for the temperature dependence of a visible single mode diode is .3nm/degree C. (Recently it was pointed out to me that visible laser diodes can have a slope of .18nm/C.) Fortunately this is not a continuous function. Otherwise we would need to hold a diode stable to within .00003C for a 20M coherence length! In-between mode hops the slope is much flatter. But even if it is .01nm/C in-between mode hops that works to a needed stability of .001C for a 20M coherence length. From my experience I can propose a rough guess of .05nm/C for inbetween mode hops.
I have used six diode lasers to make holograms and tried a few experiments as well. Everything I know about diodes was the result of a collaboration between Jonathan Head and myself. As well as some rather timely help from Tom B. and alt.lasers Jonathan is a tireless experimenter and makes very bright and clean holograms.
This is my old standby laser. When I am having fits with my designs I go back to this one. It does not give me a good hologram every time but it does more often than not. It is a Laser Max MDL660-35 with a heat sink I made. Now that the new TEC laser is complete I really never use it.
Click on image to get a CAD file.
You will need a plug-in to view .dwf files. I use the auto desk one called Whip. If you right click you can zoom to see the details. If you import this into your CAD software it retains all of the features of the original drawing.
In an effort to stabilize the temperature with low tech methods. Jonathan and I decided to use a water heatsink. I made up two of these and insulated them with 1" of Armeflex neoprene insulation. While this test provided us with proof that temperature issues were our problem there was no way to make a hologram with the heatsink. The fringes would not even settle for a second. Jonathan made a much larger version and made the diode mount go into the water from the top. He has been able to get enough stability to make holograms with his new water based heatsink.
Click on image to get a CAD file.
This is the TEC heatsink I worked for months on. I think with the help of the forum we decided that the heatsink did not have enough mass and was too sensitive to ambient changes in temperature.
This laser is the test bed for all of my tuning and experiments (Now happily retired). The heatsink is a 4"x4"x1/4" aluminum plate. One side was lapped on a glass plate to 600 grit for the thermal assembly. It will make a hologram but I have to use a beam spreader so it is not the easiest for a quick single beam test. It is a Thorlabs Constant Current Driver with a Analog Technologies TEC driver in the prototype board. I had to add a 10 turn pot to the prototype board.
This is the latest laser I have been working on. The insulation has been removed for the photo. I'll post results when I have them. The base plate and cover act as the heat sink, rf shield and dust cover. This is the same circuit as above repackaged.
These are the prints.
Well, this is all of the pictures I have for now....
To look inside a diode look here.
Please free to contact me at: firstname.lastname@example.org
Copyright © 2002 Colin Kaminski